Please, please, please tell me that you have decided to write this. Because otherwise, I might be forced to write this. It's a really fascinating concept, with the potential for all kinds of world-build-y fun and exploration of ramifications (my specialty!!)...

The closest I've seen would be the Step-Brothers series (it's recced over on crack_van, if you want to read it), in which Mary doesn't die and leave Tom an orphan. She survives, and eeks out a living on Knockturn Alley until someone pushes her into talking to an old classmate who's also stuck doing the single-parent thing (Rubeus Hagrid's father). It's a fascinating series, and like I said, probably the closest I've really seen to what you're talking about.

The only other one I've run into was where Ginny, after watching just about everyone die, goes back in time and encounters Tom in his 5th year. Once there, she presents a counterbalance and positive influence, and while events aren't massively changed, they are shifted just enough to prevent the rise of the Voldemort who destroyed her world.